Histological and biomechanical studies of hydroxylapatite implant

Bony defects measuring 12x12x12 millimeters in size were created bilaterally over the proximal end of the femurs of 10 Macaca cyclopis monkeys and implanted with particulate hydroxylapatite. The animals were sacrificed according to a predetermined sequence of post-operative 1, 2, 3, 6, and 8 months. The specimens taken from the implant sites in the two femurs of each animal were chosen randomly either for histological observation or for compressive strength tests under vertical loads. Histological pictures showed only loose fibrous tissue over the interparticular space of the implant site at the first month after surgery. Woven bone could be observed over the basal part of the implant site at the second month. New bone formed over the central part of the implant site at the third month after surgery. Continuing maturation of the bone tissue could be observed over the interparticular space of the implant site until the late stages at the 6th and 8th month after surgery. The results of compressive strength tests were compatible with the histological observations. The value of the elastic modulus was low initially at the first month, but increased gradually with time due to organization of fibrous tissue, condensation of HA particles, and ossification as well as maturation of bone tissue. The value of elastic modulus at the third month was equal to that of cortical bone tissue and even two-fold higher than that of the cortical bone at the eighth month after surgery. These results provide valuable information about the compressive strength of the HA implant in host tissue.